Bipolar transistors are conventional, popular amplifying components for power amplifiers for amplifying, for example, signals. For example, in a bipolar transistor used as an amplifying component, the base-emitter ON voltage (VBE) drops at high temperatures. So, the collector current increases with a rise in ambient temperature if the bias voltage applied to the base is constant.
To compensate this particular temperature characteristic to achieve stability of the power amplifier, the conventional power amplifier includes a “VBE-controlled voltage source circuit” as a temperature compensation circuit in a bias circuit supplying a bias voltage to the amplifying component of the power amplifier. The VBE-controlled voltage source circuit produces an output voltage according to the VBE of a transistor included in the circuit. This type of power amplifier including the VBE-controlled voltage source circuit is disclosed in, for example, Japanese Unexamined Patent Publication (Tokukai) 2002-009558 (published Jan. 11, 2002).
FIG. 24 is a schematic circuit diagram of the power amplifier disclosed in Tokukai 2002-009558. A power amplifier 100 includes, as illustrated in FIG. 24, an amplification transistor Tr101, an input matching circuit 102, an output matching circuit 103, a resistor R101, and a bias circuit 110. The base of the amplification transistor Tr101 is connected to an input signal terminal 101 of the power amplifier 100 via the input matching circuit 102. The collector of the amplification transistor Tr101 is connected to an output signal terminal 104 of the power amplifier 100 via the output matching circuit 103. In the power amplifier 100 structured that way, a high frequency signal fed at the input signal terminal 101 goes through the input matching circuit 102, amplified by the amplification transistor Tr101, goes through the output matching circuit 103, and output at the output signal terminal 104. Furthermore, the base of the amplification transistor Tr101 is connected via the resistor R101 to the bias circuit 110. To the collector of the amplification transistor Tr101 is connected a power supply terminal 105 supplying a bias voltage to the amplification transistor Tr101.
The bias circuit 110 includes transistors Tr102, Tr103, Tr104, Tr105, resistors R102, R103, power supply terminals 106, 108, and a control input terminal 107. The transistors Tr103, Tr104, the resistors R102, R103, the control input terminal 107, and the power supply terminal 108 constitutes a VBE-controlled voltage source circuit 109.
The transistor Tr102 supplies a bias current to the amplification transistor Tr101 according to a control input voltage that is fed to the bias circuit 110 as a control signal. The emitter of the transistor Tr102 is connected via the resistor R101 to the base of the amplification transistor Tr101. The resistor R101 is a stabilization resistor (ballast resistor) for the prevention of thermal-runaway of the amplification transistor Tr101. The collector of the transistor Tr102 is connected to a power supply terminal 106.
The transistor Tr105 controls the bias current. Accordingly, the power amplifier 100 is able to regulate in some cases the collector current in the presence of variations in the control input voltage. The collector of the transistor Tr105 is connected to the emitter of the transistor Tr102. The emitter of the transistor Tr105 is grounded. The transistor Tr105 may be omitted depending on the structure of the bias circuit 110.
The VBE-controlled voltage source circuit 109 has functions of lowering the sensitivity of the bias current of the power amplifier 100 to variations in the control input voltage and reducing temperature-induced variations in the collector current of the amplification transistor Tr101.
Specifically, the transistors Tr103 and Tr104 are cascaded to compensate temperature characteristics of the power amplifier 100. The collector of the transistor Tr103 is connected to the base of the transistor Tr104. Those collector and base are connected to the control input terminal 107 via the resistor R102. The power amplifier 100 receives, at the control input terminal 107, a control input voltage as a control signal from an external circuit (not shown). The base of the transistor Tr103 is connected to the emitter of the transistor Tr104. Those base and emitter are connected to the base of the transistor Tr105 and also grounded via the resistor R103. The collector of the transistor Tr104 is connected to the power supply terminal 108. In addition, in Tokukai 2002-009558, the power supply terminals 106, 108 are connected to the same external power supply.
In the structure, the output voltage of the VBE-controlled voltage source circuit 109 is equal to the sum of the VBE of the transistor Tr103 and the VBE of the transistor Tr104, that is, about twice the VBE. The base voltage of the transistor Tr104 acts also as the base voltage of the transistor Tr102. To obtain a constant collector current for the amplification transistor Tr101, the bias voltage applied to the base of the transistor Tr102 should be reduced with in temperature rise. Meanwhile, the output voltage (twice the VBE) of the VBE-controlled voltage source circuit 109 tends to decrease with temperature rise. Thus, the power amplifier 100 can restrain variations in the collector current induced by temperature.
Japanese Unexamined Patent Publication 7-200086/1995 (Tokukaihei 7-200086; published Aug. 4, 1995) discloses a reference current circuit incorporating a current mirror circuit called a Nagata current mirror circuit. In that type of current mirror circuit, the mirror current increases with an increase in the reference current, reaches a peak at a certain reference current, and decreases at higher reference currents. Tokukaihei 7-200086 enables the reference current circuit to change its role and operate as a reference voltage circuit or double as a reference voltage circuit, without adding to circuit size, by applying the Nagata current mirror circuit to the reference current circuit.
New mobile phones and communications devices used as wireless network devices need a power amplifier which exhibits reduced temperature dependence to improve wireless transmission capability, in particular, a power amplifier with very small gain variations with temperature. The power amplifier 100 of Tokukai 2002-009558 shown in FIG. 24 exhibits only small changes with temperature in the collector current of the amplification transistor Tr101, but allows decreases in the gain of the power amplifier 100 at high temperatures.
Tokukai 2002-009558 discloses another structure: there is provided in the bias circuit 110 an additional bias circuit that includes a second control input terminal other than the control input terminal 107, a resistor, and two current fine-adjusting transistors. The collector and emitter of one of the current fine-adjusting transistors are connected to the bias circuit 110. The base of this current fine-adjusting transistor is connected to the collector and base of the other current fine-adjusting transistor. The emitters of the two current fine-adjusting transistors are connected together.
The structure enables fine adjustment of the bias current by adjusting the control input voltage at the second control input terminal in an ON state while switching on/off the bias current according to the input signal fed from the control input terminal 107.
However, the two current fine-adjusting transistors of the bias circuit additionally provided in the bias circuit 110 constitute a plain current mirror circuit. Therefore, with this structure, it is still difficult to restrain gain variations in the power amplifier.
Tokukaihei 7-200086 does not disclose a Nagata current mirror circuit being adopted as a current source in the VBE-controlled voltage source circuit to regulate the temperature dependence and control input voltage dependence of the reference current circuit. Tokukaihei 7-200086, again, does not disclose the combination of the VBE-controlled voltage source circuit and the Nagata current mirror circuit being used for gain compensation in the power amplifier 100.